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Magnetocaloric effect at the second-order magnetic transition

Magnetic materials exhibiting a large magnetocaloric effect (MCE) have recently gained strong interest due to their potential use in magnetic refrigeration technology. The MCE is intrinsic to all magnetic materials, being particularly high in the vicinity of 1st- or 2nd-order magnetic phase transitions. However, to induce 1st-order transition energy must be spent to overcome the potential barrier between the two phases. This leads to intrinsic irreversibilities in both entropy

and adiabatic temperature changes which can drastically reduce cooling efficiency. In contrast, 2nd-order magnetic transitions are continuous, which typically results into lower but fully reversible entropy changes. For instance, Gd, which has been used as a benchmark material in magnetic refrigerator, shows a 2nd-order ferro-to-paramagnetic transition with high entropy change due to a large magnetic moment (7.5 μB/atom). However, the high cost of Gd prohibits its use in commercial applications.

In contrast to rare-earth based materials, much cheaper and more environmentally friendly candidates for cooling applications were found within the family of Ni-Mn based Heusler alloys. Initial interest in this group of materials was focused in the presence of magneto-structural 1st-order martensite-austenite transitions, leading to extremely large anisotropy, entropy and adiabatic temperature changes. However, the martensitic phase transition is crossed at a high energy cost which translates into large hysteresis losses and low efficiency for cooling applications. It is necessary to drastically reduce thermal hysteresis to make real use of these materials. Alternatively, we show how the flexibility of the Heusler family can be used to optimize reversible 2nd-order magnetic phase transitions for magnetocaloric applications.